1. Field of the Invention
This invention relates to a radio communication system and a radio communication apparatus using a frequency hopping method.
2. Description of the Related Art
Conventionally, a radio communication apparatus and a control station for controlling the radio communication apparatus perform communication using a predetermined frequency dedicated to control and another predetermined frequency dedicated to communication.
When the control station and the radio communication apparatus do not perform communication, the control station intermittently transmits control information at the frequency dedicated to control at a predetermined time interval, and the radio communication apparatus intermittently receives the control information at a predetermined time interval.
Before starting the intermittent reception, the radio communication apparatus performs a receiving operation until the control information can be received at one of the frequencies dedicated to control. When the radio communication apparatus becomes capable of receiving the control information, it reads the frequency used by the control station for the intermittent transmission and the interval of the intermittent transmission, and then starts the intermittent reception using the obtained information.
The interval of the intermittent reception is set to be shorter as the radio communication apparatus is more frequently used, because of the following reason.
That is, since a call-receiving operation and the like frequently occur in a radio communication apparatus which is frequently used, the operability of the apparatus is improved if the control frequency can be frequently received. On the other hand, since a call-receiving operation and the like seldom occur in a radio communication apparatus which is less frequently used, priority is given to a reduction in the amount of power consumption of the battery rather than improvement in operability.
When the radio communication apparatus becomes incapable of receiving the control information during the intermittent reception, the apparatus performs the same operation as when starting the intermittent reception in order to resume the intermittent reception.
Since the frequency for control used by the radio communication apparatus is predetermined, a time period until the control information can be received when starting the intermittent reception is very small.
A frequency hopping method is used as one of radio communication methods.
When performing communication using the frequency hopping method, the control station checks the states of all frequencies which can be used in the system, and provides a basic hopping pattern to be used in the system by selecting frequencies having an excellent radio-wave environment.
The control station then transmits a synchronizing frame and control information so that a terminal station can obtain synchronism at every frequency of the basic hopping pattern. When the terminal station has received the synchronizing frame at one of the frequencies of the basic hopping pattern, it performs intermittent reception by performing a receiving operation every time the synchronizing frame is transmitted at that frequency. Thus, the terminal station is synchronized with the control station, and performs control in accordance with the control information.
That is, a receiving operation is performed at every cycle of the hopping pattern.
When the control station has detected that the environment of the frequency used in the basic hopping pattern is degraded, it changes the frequency to be used in the basic hopping pattern to an unused frequency after determining that the environment of that frequency is excellent.
The operation of the intermittent reception and the processing of changing the hopping pattern in the conventional frequency hopping method will now be described with reference to FIGS. 15 through 17.
FIG. 15 is a flowchart illustrating the operation of a conventional control station.
In FIG. 15, processing of measuring the radio-wave environment for all frequencies capable of being used in the system is performed in step J11. In step J12, a basic hopping pattern (HP) is determined by selecting frequencies having excellent radio-wave environment. In J13, ordinary processing of transmitting a synchronizing frame for all frequencies of the basic hopping pattern is performed while switching the frequency. During this processing, in J14, the radio-wave environment of the frequencies used in the basic hopping pattern is measured. If a need for a change in the hopping pattern is determined in step J15 as the result of the measurement, the process proceeds to step J16, where the hopping pattern to be used is changed irrespective of the operation of the terminal station.
FIG. 16 is a flowchart illustrating the operation of the terminal station.
During intermittent reception at a frequency by the terminal station, in step J22, a predetermined time period (corresponding to one cycle of the hopping pattern) is counted in step J22 in order to prepare for the reception of a synchronzing signal from the control station at every predetermined time interval. If the lapse of the predetermined time period has been detected in step J23, the process proceeds to step J24, where the state of the receiving operation is controlled in order to confirm synchronism. If proper synchronism is confirmed in step J25, the process returns to step J21 and the intermittent reception is continued.
If lack of synchronism is confirmed in step J25, the process proceeds to step J26, where it is determined if the number of times of the lack of synchronism reaches a predetermined number. If the result of the determination in step J26 is "no" , the process returns to step J21 and the intermittent reception is continued. If the result of the determination in step J26 is affirmative, the process proceeds to step J27, where waiting is performed at a frequency for intermittent reception. Then, it is determined if synchronism can be obtained by receiving again a synchronizing frame during a predetemined time period (steps J28 and J29).
When synchronism cannot be obtained after the predetermined time period, the process proceeds to step J30, where the power supply is again started to resume the intermittent reception.
FIG. 17 illustrates the operation sequence of the conventional system.
In steps J31 and J32, while the control station transmits a synchronizing frame (CNT) to the terminal station using hopping frequencies f1-f10, the terminal station performs intermittent reception at the frequency f1 based on the synchronizing frame. In FIG. 17, a broken line indicates that the synchronizing frame is transmitted during the time period of f1-f10. In steps J33 and J34, when the control station uses f1-f20 by changing the hopping pattern, the terminal station which performs intermittent reception at the frequency f1 cannot receive the synchronizing frame transmitted at the frequency f11.
When the synchronizing frame cannot be received a predetermined number of times (twice in the present example), the intermittent reception is interrupted, and waiting is performed at the frequency f1 used for the intermittent reception. That is, when the synchronizing frame cannot be received because the timing for performing intermittent reception shifts, the synchronizing frame can be received by interrupting the intermittent reception and waiting at the frequency used for the intermittent reception.
When the synchronizing frame cannot be received even by waiting at the frequency used for the intermittent reception, starting processing is performed in step J36, and the processing of acquiring the hopping pattern as when the power supply is turned on is performed. That is, waiting is performed at an arbitrary frequency f30. If the synchronizing frame cannot be received even in this state, the frequency to be received is changed in step J37, and waiting is performed at the frequency f11. When the synchronizing frame from the control station can be received in step J37, the terminal station acquires the hopping pattern. When the acquisition of the hopping pattern has been completed in step J38, the terminal station determines the frequency f11 for performing intermittent reception, and intermittent reception is performed in step J39.
In the above-described conventional approach, however, when the terminal which performs communication using the frequency hopping method performs intermittent reception, since the intermittent reception is performed at an arbitrary frequency at every period of the hopping pattern, even a terminal which is frequently used cannot perform intermittent reception at an interval shorter than one period of the hopping pattern.
On the other hand, even a terminal which is less frequently used cannot lengthen the interval of intermittent reception.
When changing the basic hopping pattern in the control station in the above-described conventional approach, the following problems are present.
In step J24 shown in FIG. 16, the terminal station performs starting processing at every predetermined time interval during the intermittent reception and receives the synchronizing signal from the control station at an arbitrary frequency. However, if the control station changes frequencies used in the basic hopping pattern for some reason, the terminal station cannot receive the synchronizing signal from the control station during the intermittent reception. In such a case, the terminal station continues the intermittent reception for a while. If the synchronizing signal cannot be received even in this state, the terminal station performs starting processing and receives the synchronizing signal. Hence, a considerable amount of time is required until a state in which the terminal station can perform communication is reached after changing the hopping pattern.
Furthermore, in an inferior frequency environment where the hopping pattern must be frequently changed, the terminal station frequently performs starting processing, thereby rapidly consuming the battery.
In addition, when changing all frequencies used in the basic hopping pattern, all terminal stations which perform intermittent reception must perform starting processing. As a result, there is the possibility that all the terminals which perform intermittent reception cannot be used, thereby degrading the reliability of the system.